1905Daily News 23 May 4/7 (OED2), America . . has two substitutes for ‘yes.’ One of them is ‘yep’ and the other is ‘yah.’

1931JacobsonMilwaukee Dial. 17 WI, Oh ya. . . is extensively used both as an exclamation and as an affirmative expression.

Ibid 19, Ya. . . one of the most common forms of yes. No matter how ungerman or how well educated everyone at some time or other uses ya for yes. A New York girl was visiting in Milwaukee for a few weeks last year. Apparently unconscious of the effects of her visit she returned to the University with a broad German Ya which try as she may she can not lose.